1. Field of the Invention
This invention relates to a new and improved micro lens array and micro deflector sub-assembly fabricated from silicon semiconductor plates processed in accordance with semiconductor microcircuit fabrication technology, metalized in part and held together in assembled relationship by glass rodding to which the silicon plates are used or otherwise secured either directly or through the medium of suitable metal mounting rings.
2. Prior Art Problem
The desirability of using a matrix of micro-electron optical elements arranged in the manner of a fly's eye lens is a now well-established fact in that such an arrangement provides large field coverage without sacrifice of resolution, large beam current, deflection sensitivity or accuracy and other desirable attributes as described in a paper entitled "Electron Beam Memories" by D. E. Speliotis, D. O. Smith, K. J. Harte and F. O. Arntz, presented at the ELECTRO/76 held at Bostom, Mass. on May 11-14, 1976 and in an article entitled "Advances in Fly's Eye Electron Optics" appearing in the Proceedings of the National Electronics Conference, vol. 23, pgs. 746-751 (1967) by S. P. Newberry et al. While the desirable characteristics of the fly's eye electron optical system are well established, as the requirements for the number of channels in the matrix increases and the linear dimensions of the matrix correspondingly decrease in efforts to increase its storage capacity and minimize the size, complexity and weight of the equipment, the problems of fabrication of fly's eye electron beam systems using known materials and fabrication techniques become increasingly difficult if not insurmountable.
In the known prior art fly's eye electron optics system heretofore available to the art as described in the above-noted National Electronics Conf. article, the micro lens array sub-assembly has been fabricated in the form of a "top hat" structure as shown in FIG. 2 of the article. In this form of micro lens array, the focusing element of the micro lens consists of an array of holes formed in thin metal plates. The thin metal plates in turn are tightly stretched and bonded to a strong metal ring and the holes are produced by a variety of methods such as drilling, punching and photo-chemical etching to mention a few. The problems encountered with these known micro lens array structures are:
(1) Photo-chemical etching of metal is expensive and does not result in lens aperture openings having required roundness, smoothness and uniformity between holes in the array. PA1 (2) While punching of holes does reduce cost substantially, and if followed by a finishing operation such as shaving, does produce uniform diameters and smooth surfaces, these procedures cannot be accomplished on a matrix of holes (lens aperture openings) in which the hole diameter equals or even approaches the optimum ratio to the spacing between holes. PA1 (3) The use of heavy metal rings to support the thin plates does not permit close spacing of the plates as the spacing between lens aperture openings (channels) is decreased to optimize density of channels and minimize size. If the "top hat" structure shown in FIG. 2 of the National Electronics Conference article is employed, while permitting close spacing between lens plates, it is expensive and uses space inefficiently, but most seriously, it prevents close approach to one side of the lens elements of neighboring elements of the overall fly's eye electron optical system. PA1 (4) If an attempt is made to avoid the above-discussed difficulties encountered with the use of thick mounting rings or the "top hat" configuration by using metal plates which are thick enough to be self-supporting, eventually the impossible condition would be reached in large arrays (e.g., arrays having lens elements numbering 128.times.128) where the plate thickness required for mechanical rigidity exceeds the spacing between the plates required for optimum electron optical performance. Additionally, thick plates are more costly to process in the fabrication of the lens aperture openings (holes), are more severely limited in hole size permitted, and are inclined to warp during bake-out temperature cycling due to built-up strains. Finally, as with thin metals, the desired optimum hole diameter to spacing between holes cannot be achieved. PA1 (1) Micro deflector systems which depend upon production of individual deflector plates which are subsequently stacked together with spacers require unreasonable tolerance control because the position error is cumulative. Single blade metal deflectors are better than metal deflectors sawed from solid stock, but they are expensive and too thin to remain straight unless placed in tension by the assembly. PA1 (2) Thin metal plates are microphonic at some resonant frequency and this resonance can be excited by the application of periodic changes in the deflection voltage such as a raster scan. PA1 (3) In micro deflector systems which use deflector bars sawed from blocks, the ceramic blocks must be sawed in the fired state (i.e., very hard) in which state they are so abrasive that even diamond tools wear rapidly and the dimensions are very difficult to hold. Thus, they are costly to produce. PA1 (1) In electron beam accessed memories, thermal match is obtained between the recording media and the micro lens array and micro deflector elements since such elements are formed of silicon and glass rodding which has a temperature coefficient of expansion very near to that of silicon. PA1 (2) The high purity and regularity of the material (single crystal silicon) permits construction of the micro lens elements by known microcircuit photoetch techniques and better quality holes and straighter edges are obtained in comparison to holes formed in metals or amorphous materials. PA1 (3) Fewer problems are encountered with the flatness of the materials. PA1 (4) It is not necessary to mount the micro lens plates on a supporting ring of substantial thickness thereby permitting closer spacing between the micro lens plates. PA1 (5) As will be explained more fully hereafter, it is possible by appropriate fabrication techniques to make bi-layer lens elements without bimetallic thermal effects thus permitting the construction of highly conductive, buttressed outer lens plates having ultra thin lens aperture openings formed on a silicon lens plate of substantial thickness and conductive layers on each of the opposite sides thereof. PA1 (6) Metalization (if needed) and bonding techniques for silicon plates are well established and proven. PA1 (7) Extreme cleanliness and stability at bake-out can be obtained for the resulting structure. PA1 (8) Polycrystalline silicon is easier to saw and metalize than ceramic thus making the problem of micro deflector bar fabrication much less costly and better controlled. PA1 (9) In addition to producing smoother more uniform lens aperture openings (holes) in silicon plates, the photochemical etching techniques used in producing the holes permit hole size to center spacing to be controlled to optimum values.
Turning attention now to the micro deflector structure for achieving fine deflection, the above-mentioned National Electronics Conference article describes a micro deflector construction which has been successfully applied to the fly's eye lens and comprises two sets of parallel conductive bars in tandem. The use of metal plates to produce the deflection bars has not been satisfactory, however, for reasons to be discussed hereafter. Sawing of bars from ceramic blocks and metalization of the ceramic bars, has produced electron optically acceptable fine deflectors but the cost has been unacceptable and the yield very low. In summary, experience with the known fine deflector sub-assembly design has taught the following lessons:
In addition to the component fabrication problems discussed above, the overall structure, i.e., the micro lens array plus micro deflector and target electrode member, has further constraints. Since a single piece of dirt can spoil an assembly for many applications, the assembled structure must either be capable of disassembly for cleaning or fabricated by techniques which leave it electron optically clean. Additionally, the assembly must not permit relative motion of the parts by environmental factors such as vibration or thermal excursions. Two of the most important applications for fly's eye type electron beam tubes are in electron beam accessed semiconductor target memories for use with computers and in microcircuit pattern fabrication. In these applications, if the target area covered is large, then temperature excursions pose a severe problem with the mixing of construction materials such as metals, ceramics and semiconductor targets each with a different temperature coefficient of expansion and pattern displacement of several microns can occur due to normal room temperature variations. Thus, it will be appreciated that the above-listed requirements can make the overall assembly of a fly's eye electron beam tube micro lens array and micro deflector a very difficult problem.
From the foregoing discussion, it would be appreciated that new materials and methods of construction of micro lens arrays and micro deflector sub-assemblies are required if the benefit of higher density, larger arrays are to be achieved for the industry.